This invention is directed to recovery of ammonia from a vaporous process stream such as, for example, the reactor effluent from a hydrocarbon ammoxidation reactor. More particularly, the invention is directed to an improved process for the recovery of ammonia contained in the effluent stream from a reactor employed in the catalytic ammoxidation of propylene, propane, isobutane or isobutylene in the production of acrylonitrile or methacrylonitrile.
The processes widely used in commercial practice for recovering the products of hydrocarbon ammoxidation generally comprise the steps of: a) contacting the effluent from an ammoxidation reactor in a quench tower with an aqueous quench liquid to cool the gaseous effluent; b) contacting the quenched effluent with water in an absorber, forming an aqueous solution comprising the ammoxidation products; c) subjecting said aqueous solution to a water extractive distillation, and d) removing a first overhead vapor stream comprising acrylonitrile or methacrylonitrile and some water from the top of the column, and collecting a liquid waste stream containing water and contaminants from the bottom of said column. Further purification of the nitrile may be accomplished by passing the overhead vapor stream to a second distillation column to remove at least some impurities from the crude nitrile, and further distilling the partially purified nitrile in a third distillation column to obtain the purified acrylonitrile or methacrylonitrile.
Hydrocarbon ammoxidation, particularly of alkanes, is typically conducted using substantial excesses of ammonia. Ammonia that is not consumed in the ammoxidation exits in the reactor effluent, together with nitrile monomer and reaction by-products, including hydrocyanic acid, cyanoalkane and the corresponding aldehyde and the like. The by-products react with nitrile monomer in the presence of ammonia, or react with one another. It is therefore necessary to separate the ammonia from the effluent stream immediately after exiting the ammoxidation reactor. Conventionally, the unreacted ammonia is captured in the quench operation, step a), by including sufficient acid in the aqueous quench liquid to neutralize the excess ammonia. The aqueous bottoms comprising the corresponding salt together with other water-soluble by-products and contaminants may be removed from the quench tower, combined with other by-product streams produced in the process and recovered or prepared for disposal in an environmentally safe manner.
Alternatively, recovering the ammonia for further use may be of considerable importance to the overall process economics. Methods for regenerating the captured ammonia for recycle have been disclosed in the art.
Great Britain Patent 222,587 discloses a process for capture of ammonia from an ammonia-containing gas mixture utilizing an aqueous phosphoric acid, an aqueous solution of ammonium dihydrogen phosphate ((NH4)H2PO4), or mixtures thereof. The ammonia is recovered by thermally decomposing the ammonium phosphate mixture. The residue is then dissolved in water to regenerate the ammonium phosphate solution.
U.S. Pat. Nos. 2,797,148 and 3,718,731 are directed to the recovery of ammonia from a process stream in the production of HCN. Generally described, the ammonia-containing gas is contacted with a 25% to 35% by weight ammonium phosphate solution having a pH of about 6 at a temperature of from 55° C. to 90° C. Ammonia regeneration is effected by contacting the resulting phosphate solution with steam.
In U.S. Pat. No. 5,895,635 there is disclosed a recovery process wherein effluent from an alkane ammoxidation reactor is passed into a quench tower and contacted with an aqueous ammonium phosphate quench solution. The quench solution will comprise monoammonium phosphate and may further comprise phosphoric acid and diammonium phosphate, and have a ratio of ammonium ions (NH4+) to phosphate ions (PO4−3) of from about 0.7 to about 1.3, at a pH of from about 2.8 to about 6. The ammonia in the process stream is absorbed in the quench solution, reacting with the monoammonium phosphate component and forming diammonium phosphate. Heating the diammonium phosphate in a separate operation decomposes the diammonium phosphate, reforming the aqueous monoammonium phosphate solution and generating a vapor stream containing ammonia for recycle or recovery.
U.S. Pat. No. 5,895,635 further discloses methods for treating the aqueous phosphate solution including subjecting the aqueous monoammonium phosphate solution to wet oxidation at an elevated temperature and pressure to remove heavy organics and other objectionable contaminants prior to recycle as quench liquid. According to patentees, excess water may also be removed as needed from the ammonium phosphate solution either with the ammonia vapor stream during the decomposition step or by passing the aqueous stream exiting the wet oxidation process through evaporation means before recycle in the quench operation. Such further treatment may significantly reduce the volume of aqueous waste requiring disposal and further reduce operating costs.
The recovery and treatment methods heretofore disclosed in the art require further improvement to overcome inefficiencies and provide a more environmentally acceptable aqueous waste stream. For example, where an ammonium phosphate quench solution is employed, the residuals stream from the quench operation may retain as much as about 5% of the recoverable ammonia present in the reactor effluent stream; contaminants that remain in aqueous solution after the ammonia stripping operation may be carried into the ammonia stream from the decomposer and then into the wastewater stream, adding to the waste disposal burden; and although wet oxidation may be very effective for destroying the heavy organic components of the quench stream, wet oxidation is costly, and its use will typically be limited to reducing the contaminants to a level acceptable for recycle, for example, by treating only a fraction of the quench stream. Because of these and further inefficiencies in the methods disclosed in the art for ammonia recovery, it is desirable to have an improved method for ammonia recovery for use, for example, in the ammoxidation processes employed for the production of nitrile monomers. This invention provides such improved methods. The improvement in the efficiency of the recovery and treatment methods provided by this invention will thus be an important advance in the ammoxidation process art.